Digital Modulator and Digital-to-Analog Conversion Techniques Associated Therewith

ABSTRACT

Some embodiments disclosed herein relate to a transmitter. The transmitter includes a digital modulator adapted to provide a digital modulated RF signal based on a multi-bit representation of data and a multi-bit representation of a carrier wave. A digital-to-analog converter (DAC) is adapted to generate an analog modulated RF signal based on the digital modulated RF signal. A resonant circuit coupled to an output of the DAC and adapted to filter undesired frequency components from the analog modulated RF signal.

BACKGROUND

Modulation is the process of varying one waveform in relation to anotherwaveform. In telecommunications, modulation is used to convey data froma transmitter to a receiver over a communication channel. For example,transmitters in cellular phones, modems, and other modern communicationdevices often use modulation to efficiently transmit data.

Although modulation schemes are widely-used in communication systems,previous transmitters have included analog circuitry for implementingthe desired analog or digital modulation techniques. However, theinventors have appreciated that analog circuitry is less than idealbecause it is inflexible and typically consumes a relatively largeamount of power. Consequently, the inventors have appreciated that it isdesirable to attempt to modulate waveforms in digital fashion to thegreatest extent possible. The use of digital circuitry is advantageousin that it often provides greater flexibility and lower powerconsumption than analog solutions. The lower power consumption, inparticular, enables battery-powered communication devices (e.g.,cellular phones) to operate for longer periods of time withoutre-charging, which is a desirable feature for many end-users.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a transmitter that includes a digitalmodulator in accordance with some embodiments.

FIG. 2 is a functional block diagram illustrating a transmitter thatincludes a digital polar modulator.

FIGS. 2A-2B show waveform diagrams in accordance with the embodiment ofFIG. 2;

FIG. 3 is a functional block diagram illustrating a transmitter thatincludes an IQ modulator.

FIG. 4 is a block diagram of a transmitter that includes a more detailedembodiment of a digital-to-analog converter.

FIG. 5 illustrates a sawtooth waveform which can be generated by thedigital-to-analog converter of FIG. 4.

FIG. 6 is a flow diagram illustrating a method in accordance with someembodiments.

FIG. 7 is a series of waveforms that show some modulation techniques inwhich analog data is modulated onto a carrier wave.

FIG. 8 is a series of waveforms that show some modulation techniques inwhich digital data is modulated onto a carrier wave.

FIGS. 9-10 illustrate examples of transmitters that include a digitalup-conversion element in accordance with some embodiments.

DETAILED DESCRIPTION

Transmitter implementations are now described with reference to thedrawings, wherein like reference numerals are used to refer to likeelements throughout. In the following description, for purposes ofexplanation, numerous specific details are set forth in order to providea thorough understanding of the implementations. It may be evident,however, that the implementations may be practiced without thesespecific details.

Whereas previous transmitters have used analog circuitry to carry outmodulation, some aspects of the present disclosure provide digitalcircuitry to carry out modulation. For example, some embodiments includea digital modulator (e.g., microprocessor or digital applicationspecific integrated circuit (ASIC)), wherein a digital-to-analogconverter (DAC) is coupled to an output of the digital modulator. Apass-band filter (e.g., LC resonant circuit) is coupled to an output ofthe DAC to attenuate unwanted frequency components in the analogwaveform provided by the DAC. As will be appreciated in greater detailfurther herein, the digital circuitry provided herein helps provideflexibility from a programming standpoint while at the same timeexhibiting favorable power consumption, thereby helping to provideend-users with a communication device that meets or exceeds theirexpectations.

FIG. 1 shows one example of a transmitter 100 in accordance with someembodiments. The transmitter 100 includes a baseband processor 102, adigital modulator 104, a digital-to-analog converter (DAC) 106, apassband filter 108, a power amplifier 110, and a radio frequency (RF)antenna 112, which are operably coupled as shown. As will be appreciatedin more detail below, the digital modulator 104 generates a stream ofmulti-bit values (which are collectively representative of a carrierwave onto which data has been modulated), and then the DAC 106 convertsthe digital data into an analog waveform suitable for transmission overthe antenna 112.

The baseband processor 102 includes a first output that provides afrequency control word 114 and a second output that provides a stream ofdigital data 116, such as I-Q data for example. The frequency controlword 114 and digital data 116 are often delivered according to asampling rate provided by the clock generator 117. The frequency controlword 114 can be a multi-bit value that corresponds, for example, to acarrier frequency multiplied by a constant value; and the stream ofdigital data 116 often specifies how the carrier frequency is to bemodulated in time.

Upon receiving the frequency control word 114 and the stream of digitaldata 116, the digital modulator 104 outputs a digital modulated RFsignal 118. The digital modulated RF signal 118 is a time-varyingmulti-bit value that is based on both the frequency control word 114 andthe digital data 116 and which changes according to the sampling rate.

The DAC 106 converts the digital modulated RF signal 118 into an analogmodulated waveform 120. Passband filter 108, which can be combined onthe same integrated circuit as the DAC 106 and which can include aresonant circuit in some embodiments, removes unwanted frequencycomponents from the analog modulated waveform 120, while allowing awanted signal 122 to pass through. The power amplifier 110 thenamplifies the wanted signal 122, thereby generating a RF signal 126 tobe transmitted over the antenna 112.

In some embodiments, the base band processor 102 may also adjust thepassband filter 108 (as indicated by optional control signal 124) toallow the wanted signal 122 to pass to the antenna 112. For example, thecontrol signal 124 may adjust a bank of capacitors in the passbandfilter 108, to “tune” the filter to allow a carrier wave frequency topass while other undesired frequencies are blocked.

It will be appreciated that because the transmitter 100 includes adigital modulator 104 rather than an analog modulator as used inprevious approaches, the transmitter 100 can be programmed to facilitatea variety of communication techniques while keeping power consumption atlow-levels. For at least this reason and/or other reasons, variousembodiments of transmitters that include a digital modulator areimprovements over those previously known.

FIG. 2 shows another embodiment of a transmitter 200 that includes adigital polar modulator 201 (e.g., digital modulator 104 of FIG. 1). Thedigital polar modulator 201 includes a cordic 202, a differentiator 204,an adder 206, a phase accumulator 208, an angle-to-amplitude conversionelement 210, and a digital multiplier 212, which are operably coupled asshown. In many embodiments, each of these components operates on clocksignal F_(s), which is received on clockline 213 (e.g., coupled to clockgenerator 117 of FIG. 1) and facilitates sampling as described furtherherein. Although FIG. 2 does not explicitly depict a pass band filter(e.g., resonant circuit), power amplifier, or antenna, it will beappreciated that these components are often included as shown in FIG.1's implementation, for example.

Referring now to FIG. 2 in conjunction with FIG. 2A, operation of thedigital polar modulator 201 is described with regards to amplitude onlymodulation where no phase modulation is present. Amplitude onlymodulation begins when a baseband processor 214 provides a frequencycontrol word 216 as well as digital data 218 in I-Q format to thedigital polar modulator 201.

The cordic 202 translates the I-Q data 218 into polar data 222, whichincludes a phase component θ(t) and an amplitude component r(t).Successive polar data values are separated by time F_(s) (i.e.,according to the sampling rate provided on clock line 213), as aresuccessive data values of the frequency control word 216. An advantageof this configuration is that it allows the baseband processor 214 tochange the channel frequency (frequency of carrier wave) by changing thefrequency control word 216.

The differentiator 204 differentiates the polar data 222, therebyproviding a multi-bit instantaneous frequency offset value 223 at thesampling rate. Each instantaneous frequency offset value 223 representsan offset of a present instantaneous frequency or phase relative to aprevious successive instantaneous frequency or phase.

At each sampling interval, the adder 206 adds the frequency control word216 with the instantaneous frequency offset value 223, thereby providingan instantaneous phase offset at 224. As shown in FIG. 2A, for amplitudeonly modulation the instantaneous phase offset 224 is approximatelyconstant, wherein successive instantaneous phase offset values areseparated by sampling interval F_(S).

The phase accumulator 208 continuously accumulates successive multi-bitinstantaneous phase offsets 224, thereby providing an instantaneousphase at the accumulator output 226. The phase accumulator 208 typicallyincludes an N-bit latch such that its output 226 exhibits a range of0≦K≦2^(N)−1, where K is an N-bit binary number stored in phaseaccumulator 208 at any time. See numeral 226 in FIG. 2A. Therefore, forsome clock cycles, when an instantaneous phase offset value 224 is addedto the N-bit binary number (K) presently stored in the phase accumulator208, the resultant N-bit binary number K exceeds 2^(N)−1, therebycausing the phase accumulator 208 to overflow. Because the accumulatoris modular in this respect, the accumulator output 226 can beinterpreted as an N-bit instantaneous phase or “angle” of a carrierwave.

The angle-to-amplitude conversion element 210, which include a memorythat stores a sine or co-sine lookup table in some embodiments, receivesthis N-bit instantaneous phase 226, and outputs a correspondingmulti-bit binary number 228. As shown in FIG. 2A (210), the angle toconversion element “maps” the instantaneous phase at accumulator output226 to an amplitude value of a digital waveform, such as a sine orco-sine waveform.

The digital multiplier 212 receives the stream of multi-bit numbers 228,and selectively modifies this stream based on the amplitude componentr(t) to produce a stream of multi-bit numbers representing anamplitude-modulated waveform 230. Thus, in the illustrated example ofFIG. 2A, a latter portion of the waveform 228 has beenamplitude-modulated to have an amplitude of zero in 230. In this way,the digital polar modulator 201 outputs a stream of multi-bit binarynumbers that vary according to sample rate F_(s), and which isrepresentative of a polar modulated waveform.

FIG. 2B shows another example wherein the digital polar modulator 201 isused to achieve an amplitude and phase modulated waveform at the output230 of the digital polar modulator 201. The digital polar modulatorcould also achieve other types of modulation in addition to those thatare illustrated.

FIG. 3 shows another embodiment of a transmitter 300 that includes adigital IQ modulator 302 (e.g., digital modulator 104 of FIG. 1). Thedigital IQ modulator 302 includes a phase accumulator 304, an angle tocosine amplitude conversion element 306 coupled to a first mixer 308, anangle to sine amplitude conversion element 310 coupled to a second mixer312, and an adder 314, all of which are operably coupled as shown.Although FIG. 3 does not explicitly depict a pass band filter (e.g.,resonant circuit), power amplifier, or antenna, it will be appreciatedthat these components are often included as shown in FIG. 1'simplementation, for example.

During operation, a frequency control word 316 is provided to the phaseaccumulator 304, which again accumulates successive frequency controlwords according to the sampling rate F_(S). The accumulated value isthen output to the cosine and sine amplitude conversion elements 306,310 (e.g., cosine and sine lookup tables, respectively). Thus, amulti-bit value indicative of a cosine amplitude is provided to thefirst mixer 308, where it is mixed with the Q-data signal 318. Anothermulti-bit value indicative of a sine amplitude is provided to the secondmixer 312, where it is mixed with the I-data signal 320. The mixedsignals are then summed at the adder 314 to generate a stream ofmulti-bit numbers on 322 representing a digital I-Q modulated waveformto be transmitted over an antenna.

FIG. 4 illustrates another transmitter 400 (e.g., transmitter 100,transmitter 200, or transmitter 300) that includes a DAC 402 having adifferential output 404 coupled to a pass band filter 406. In theillustrated embodiment, the pass band filter 406 comprises a resonantcircuit, such as an L-C circuit made up of an inductor 408 and acapacitor 410. In other embodiments, the pass band-filter 406 couldcomprise a surface acoustic wave (SAW) filter, bulk acoustic wave (BAW)filter, duplexer, or some other type of resonant circuit. In any case,the filter is adapted to remove unwanted frequency components from ananalog waveform provided at the output 404 of DAC.

Upon receiving a stream of multi-bit values representing a modulatedwaveform at input 412, the DAC 402 converts the stream of multi-bitvalues into an analog waveform suitable for transmission at output 404.To facilitate this behavior, the DAC may include a decoder 414 and firstand second variable current sources (416, 418, respectively).

The first variable current source 416 is coupled to a first leg 420 ofthe differential output 404, and the second variable current source 418is coupled to a second leg 422 of the differential output 404. Eachvariable current source is made up of a plurality of individuallyselectable current sources. The individually selectable current sourcescomprise respective switching elements 424 a-424 f (e.g., MOStransitors) in series with respective current sources 426 a-426 f (e.g.,MOS transistors). Each switching element can include a gate operablycoupled to a different bit line of a bus 428. For purposes ofillustration, FIG. 4 shows each variable current source consisting ofthree individually selectable current sources; however, it will beappreciated that other embodiments may include any number ofindividually selectable current sources. In addition, the currentsources 426 a-426 f can be constant current sources or variable currentsources, depending on the implementation. For example, rather then usinga switch 424 a in series with current source 426 a, other embodimentscan replace these two elements with a single variable or switchablecurrent source.

To highlight one example of how FIG. 4's transmitter 400 could produceat its differential output 404 a saw-tooth waveform 500 as shown in FIG.5, reference is made to Table 1 below. It will be appreciated that, inpractical implementations it would be difficult to generate sawtoothwaveform 500 with the DAC 402. This is because the presence of filter406, which is of a passband variety, would typically block many of thefrequency components necessary for such a sawtooth wave. Nonetheless,FIG. 5's sawtooth example is discussed below to illustrate how thecurrent sources 426 a-426 f and transistors 424 a-424 f collectivelyoperate to achieve time varying analog waveforms at the differentialoutput 404.

With that said, in Table 1's example, a three-bit binary signal isprovided at the output 412 of digital modulator, and the decoder 414converts the three bit binary signal to a six-bit binary signal on bus428. These multi-bit binary signals change in time (T0-T9) toselectively couple individual current sources 426 a-426 f to thedifferential output 404 to generate the sawtooth analog waveform 500.

TABLE 1 Modulated Decoder Condition of Analog waveform output SwitchingOutput value on word on elements Value on Time 412 428 424a-424f 404 T0000 111_111 ON: none 0 OFF: 424a-424f T1 001 011_111 ON: 424a 0.33 OFF:424b-424f T2 010 001_111 ON: 424a, 424b 0.67 OFF: 424c-424f T3 011000_111 ON: 424a-424c 1.0 OFF: 424d-424f T4 010 001_111 ON: 424a, 424b0.67 OFF: 424c-424f T5 001 011_111 ON: 424a 0.33 OFF: 424b-424f T6 000111_111 ON: none 0 OFF: 424a-424f T7 101 111_011 ON: 424f −0.33 OFF:424a-424e T8 110 111_001 ON: 424e-424f −0.67 OFF: 424a-424d T9 111111_000 ON: 424d-424f −1 OFF: 424a-424c

For purposes of simplicity the example in Table 1 assumes that theindividual current sources 426 a-426 f are substantially identical. Inother embodiments, however, the current sources 426 a-426 f comprisetransistors with different length to width ratios that supply differentcurrents. In addition, although Table 1 and FIG. 5 show a saw-tooth wavefor purposes of illustration, it is to be appreciated that the analogwaveform that is output will often be a modulated waveform (e.g.,frequency modulated, amplitude modulated, or phase modulated). Note thatall digital modulation schemes (e.g., phase shift keying (PSK),quadrature amplitude modulation (QAM), orthogonal frequency divisionmultiplexing (OFDM)) can make use of phase and amplitude modulation, andthe disclosed techniques are not limited to analog frequency oramplitude modulation.

FIG. 6 shows a methodology in accordance with some aspects of thisdisclosure. While this method is illustrated and described below as aseries of acts or events, the present disclosure is not limited by theillustrated ordering of such acts or events. For example, some acts mayoccur in different orders and/or concurrently with other acts or eventsapart from those illustrated and/or described herein. In addition, notall illustrated acts are required and the waveform shapes are merelyillustrative and other waveforms may vary significantly from thoseillustrated. Further, one or more of the acts depicted herein may becarried out in one or more separate acts or phases.

FIG. 6 starts at 602 when a multi-bit representation of an RF signal isgenerated. The multi-bit representation of the RF signal often changesin time according to a sampling rate. Generation of the multi-bit RFsignal may include several sub-blocks. For example FIG. 6 shows anexample that includes four sub-blocks (604, 606, 608, and 610) thatcollectively achieve amplitude and phase modulation of data onto acarrier wave. Other embodiments could include other sub-blocks dependingon the type of modulation employed.

At 604, the method 600 provides a multi-bit frequency control word. Themulti-bit frequency control word can change according to a samplingrate, although it is often constant for a considerable amount of timeduring which the transmitter transmits over a given frequency channel.For example, FIG. 2's previously discussed embodiment of a transmitter200 disclosed a frequency control word 216 which changed in timeaccording to sampling rate F_(S).

At 606, the method 600 provides a multi-bit representation of phase datathat changes in time according to the sampling rate. For example, FIG.2's previously discussed embodiment of transmitter 200 disclosed polardata 222 that included phase data (θ(t)) that changed in time accordingto sampling rate F_(S).

At 608, the method 600 digitally provide a multi-bit phase-modulatedsignal based on the phase data and the frequency control word, whereinthe multi-bit phase modulated signal changes in time according to thesampling rate. For example, FIG. 2's previously discussed embodiment oftransmitter 200 disclosed phase modulated data 228 that changed in timeaccording to sampling rate F_(S).

At 610, the method 600 alters the phase-modulated data based onamplitude data to provide a multi-bit amplitude-and-phase modulatedsignal that changes in time according to the sample rate. For example,FIG. 2's previously discussed embodiment of transmitter 200 disclosedamplitude-and-phase modulated data 230 that changed in time according tosampling rate F.

At 612, the method 600 converts the multi-bit representation of the RFsignal into a time-varying analog RF signal. This conversion is carriedout by a digital-to-analog converter (DAC) having a resonant circuit(e.g., LC circuit) coupled to its output.

At 614, the method uses the resonant circuit to remove unwantedfrequency components from the analog RF signal. Because this method 600modulates waveforms in digital fashion, this method tends to providegreater flexibility and lower power consumption than correspondinganalog solutions.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

To provide a few examples of basic modulation techniques that may beemployed by transmitters in accordance with some embodiments, thisdisclosure now turns to FIGS. 7-8. More particularly, FIG. 7 showsanalog data (e.g., analog voice data) being modulated onto a carrierwave according two different types of modulation (amplitude andfrequency modulation); while FIG. 8 shows digital data (i.e., digitalmessage “010011 . . . ”) being modulated onto a carrier wave accordingto three different types of modulation (amplitude, frequency, and phaseshift keying). FIGS. 7-8 and the description below are intended toprovide a context for some ways in which modulation can be used totransmit data over a communication channel. However, it will beappreciated that these figures are not intended to in any way encompassall types of modulations which may be used, and are not to be construedin a limiting manner.

FIG. 7 shows analog data 702 being modulated onto a carrier wave 704according to two different types of modulation (amplitude modulation andfrequency modulation). In amplitude modulation, an amplitude modulatedwave 706 has a time-varying amplitude indicative of the analog data 702.In frequency modulation (which can be also be thought of as phasemodulation in some contexts), the frequency modulated wave 708 has atime-varying frequency indicative of the analog data 702. Modulationwith analog data can also include combinations of amplitude, frequencyand phase modulation.

In digital transmission (FIG. 8), one or more bits of digital data 802are transmitted according to a regularly repeating symbol period. Itwill be appreciated that although each symbol in the illustratedexamples of FIG. 8 conveys only a single bit of data, in otherimplementations a symbol can convey multiple bits of data. In addition,many implementations (e.g., QAM, EDGE) may use combinations of thesemodulation techniques. For example, some implementations may use bothamplitude and frequency modulation, while other implementations may useboth amplitude and phase modulation.

In amplitude shift keying (ASK) modulation, for example, a transmittercan modulate the digital data 802 onto a carrier wave 804 such that anASK-modulated waveform 806 exhibits a time-varying amplitude indicativeof the digital data 802. Thus, during the first symbol period of 0 toTS₁, the ASK-modulated waveform 806 has an amplitude of approximatelyzero (e.g., representing a “0” data state). During the second symbolperiod of TS₁ to TS₂, the ASK-modulated waveform 806 has an amplitude ofapproximately one relative to that of the carrier wave (e.g.,representing a “1” data state). Other symbol periods TS₃, TS₄, TS₅, andTS₆ show similar encoding.

In frequency shift keying (FSK) modulation, the transmitter can modulatethe digital data 802 onto the carrier wave 804 such that a FSK-modulatedwaveform 808 exhibits a time-varying frequency indicative of the digitaldata 802. Thus, during the first symbol period 0 to TS₁, theFSK-modulated waveform 808 has a first frequency f₁ (e.g., representinga “0” data state). During the second symbol period TS₁ to TS₂, thefrequency modulated waveform 808 has a second frequency f₂ (e.g.,representing a “1” data state), and so on.

In phase shift keying (PSK) modulation, a transmitter can modulate thedigital data 802 onto the carrier wave 804 such that a PSK-modulatedwaveform 810 exhibits a time-varying phase indicative of the digitaldata 802. Thus, during the first symbol period 0 to TS₁, thePSK-modulated waveform 810 is completely in-phase with the carrier wave804 and thus has a zero degree phase offset relative to the carrier wave(e.g., representing a “0” data state). During the second symbol periodTS₁ to TS₂, the PSK-modulated waveform 810 is 180° out of phase with thecarrier (e.g., representing a “1” data state).

Whatever type of modulation is used by a transmitter, the correspondingreceiver can “decode” the modulated waveform by comparing the receivedwaveform to the expected carrier wave, which is generally specifiedprior to communication. In this way, analog or digital data can bemodulated onto a carrier wave to convey a message from transmitter toreceiver.

It will be appreciated that digital modulators in accordance with thisdisclosure can take many forms in addition to those disclosed above. Forexample, FIGS. 9-10 show some transmitter embodiments where a digitalup-conversion or up-sampling element is included between an angle toconversion element and a DAC.

More particularly, FIG. 9 shows an embodiment of a transmitter 900 thatincludes digital polar modulator 902 with components similar to that ofFIG. 2, except that FIG. 9's digital polar modular 902 includes anup-conversion element 904. In FIG. 9's embodiment, a digital modulatedwaveform is provided at output 230 with a first frequency. Theup-conversion element 904 increases the frequency of this digitalmodulated waveform such that an up-converted modulated waveform atoutput 906 includes a second frequency that is higher than the firstfrequency. In one embodiment, for example, the digital modulatedwaveform at output 230 has a frequency of about 100 MHz and theup-converted modulated waveform at output 906 includes a frequency ofabout 900 MHz, although a wide range of other frequencies are alsopossible.

The up-conversion element 904 can accomplish this frequency upshift invarious manners. For example, the up-conversion element 904 can insertadditional sample values between successive multi-bit values from output230. This results in alias signals on output 906, some of which arewanted signals and some of which are unwanted signals. By using adigital filter 908, the wanted signals (which represent a waveform witha higher frequency than that of the multi-bit values on output 230) canbe selected, and the unwanted signals can be blocked. Thus, only thewanted signals are passed through to the output 910 and to the DAC. Forexample, the output 230 could have a sampling rate of 250 millionsamples per second (MSPS) and represent a modulated wave at 100 MHz. Byusing four times oversampling, the up-conversion element 904 achieves asampling rate of 1000 MSPS and an aliasing signal at 900 MHz (as well asother unwanted frequencies). The digital filter 908 then passes thewanted 900 MHz signal to the DAC, while blocking the other unwantedfrequencies.

FIG. 10 shows an embodiment of a transmitter 1000 that includes adigital IQ modulator 1002 with components similar to that of FIG. 3,except that FIG. 10's digital IQ modular 1002 includes up-conversionelement 1004. In FIG. 10's embodiment, an IQ digital modulated waveformis provided at output 322 with a first frequency. The up-conversionelement 1004 increases the frequency of the IQ digital modulatedwaveform on 302 such that up-converted modulated waveform at output 1006exhibits a second frequency that is higher than the first frequency.Because this up-conversion can cause aliasing, a digital filter 1008 canalso allow a wanted signal 1010 to pass to the DAC while blockingunwanted signals.

Certain terms are used throughout the specification to refer toparticular system components. As one skilled in the art will appreciate,different companies can refer to a component by different names. Thisdocument does not intend to distinguish between components that differin name but not function herein. In this document the terms “including”and “comprising” are used in an open ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . ” Also, theterm “couple” (and variations thereof) is intended to mean either anindirect or direct electrical connection. Thus, if a first element iscoupled to a second element, that connection may be a direct electricalconnection, or may be an indirect electrical connection via otherelements and connections. Although various approximately numeric valuesare provided herein, these numeric values are merely examples should notbe used to limit the scope of the disclosure.

Also, although the disclosure has been shown and described with respectto one or more implementations, equivalent alterations and modificationswill occur to others skilled in the art based upon a reading andunderstanding of this specification and the annexed drawings. Thedisclosure includes all such modifications and alterations and islimited only by the scope of the following claims. In particular regardto the various functions performed by the above described components(e.g., elements and/or resources), the terms used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary implementations of thedisclosure. In addition, while a particular feature of the disclosuremay have been disclosed with respect to only one of severalimplementations, such feature may be combined with one or more otherfeatures of the other implementations as may be desired and advantageousfor any given or particular application. In addition, the articles “a”and “an” as used in this application and the appended claims are to beconstrued to mean “one or more”.

Furthermore, to the extent that the terms “includes”, “having”, “has”,“with”, or variants thereof are used in either the detailed descriptionor the claims, such terms are intended to be inclusive in a mannersimilar to the term “comprising.”

1. A method of generating a radio-frequency (RF) signal in atransmitter, comprising: generating a multi-bit representation of the RFsignal, where the multi-bit representation of the RF signal changes intime according to a sampling rate; and converting the multi-bitrepresentation of the RF signal into a time-varying analog RF signal byusing a digital to analog converter (DAC) having a resonant circuitcoupled to an output of the DAC.
 2. The method of claim 1, where adigital polar modulator generates the multi-bit representation of the RFsignal.
 3. The method of claim 1, wherein generating the multi-bitrepresentation of the RF signal comprises: providing a multi-bitrepresentation of a frequency control word; providing a multi-bitrepresentation of phase data that changes in time according to thesampling rate; providing a multi-bit phase-modulated signal based onboth the frequency control word and the phase data, wherein themulti-bit phase modulated signal changes in time according to thesampling rate.
 4. The method of claim 3, wherein generating themulti-bit representation of the RF signal further comprises: alteringthe multi-bit phase-modulated signal based on amplitude data to providea multi-bit amplitude-and-phase-modulated signal that changes in timeaccording to the sampling rate.
 5. A transmitter, comprising: a digitalmodulator to provide a digital modulated RF signal based on both amulti-bit representation of data and a multi-bit representation of afrequency control word; a digital-to-analog converter (DAC) to generatean analog modulated RF signal based on the digital modulated RF signal;and a resonant circuit coupled to an output of the DAC, the resonantcircuit to filter undesired frequency components from the analogmodulated RF signal.
 6. The transmitter of claim 5, where the output ofthe DAC is a single-ended output.
 7. The transmitter of claim 5, wherethe output of the DAC is a differential output.
 8. The transmitter ofclaim 7, where the DAC comprises: a first variable current sourcecoupled to a first leg of the differential output; and a second variablecurrent source coupled to a second leg of the differential output. 9.The transmitter of claim 8: where the first variable current sourcecomprises multiple current sources coupled to the first leg of thedifferential output; where the second variable current source comprisesmultiple current sources coupled to the second leg of the differentialoutput; wherein the multiple current sources in the first variablecurrent source and the multiple current sources in the second variablecurrent source are arranged to cooperatively deliver different currentsto the first and second legs of the differential output for differentvalues of the digital modulated RF signal.
 10. The transmitter of claim8, where the first variable current source comprises: a first switchingelement in series with a first current element, where the firstswitching element is coupled between the first current element and thefirst leg of the differential output; and a second switching element inseries with a second current element, where the second switching elementis coupled between the second current element and the first leg of thedifferential output; wherein the first and second switching elements arearranged to cooperatively deliver different currents to the first leg ofthe differential output for different values of the digital modulated RFsignal.
 11. The transmitter of claim 5, where the resonant circuitcomprises at least one of the following three elements: asurface-acoustic wave (SAW) filter, a bulk acoustic wave (BAW) filter,or a duplexer.
 12. The transmitter of claim 5, where the resonantcircuit comprises an LC circuit including an inductor in parallel with acapacitor.
 13. The transmitter of claim 12, where the capacitor in theLC circuit comprises a bank of capacitors that are arranged to providethe LC circuit with an adjustable capacitance.
 14. The transmitter ofclaim 5, where the digital modulator comprises a digital I-Q modulatorthat receives data in I-Q format to produce the digital modulated RFsignal.
 15. The transmitter of claim 14, where the digital I-Q modulatorcomprises: a phase accumulator to provide an accumulated value based onsuccessive frequency control words; an angle to cosine amplitudeconversion element to convert the accumulated value to a cosineamplitude value; a first mixer to mix the cosine amplitude value with aQ-data signal to provide a first mixed value; an angle to sine amplitudeconversion element to convert the accumulated value to a sine amplitudevalue; a second mixer to mix the sine amplitude value with an I-datasignal to provide a second mixed value, where the I-data signal is 90°phase shifted with respect to the Q-data signal; and an adder to sum thefirst mixed value and the second mixed value.
 16. The transmitter ofclaim 5, where the digital modulator comprises a digital polar modulatorthat receives data in polar format to produce the digital modulated RFsignal.
 17. A circuit that includes a digital modulator, the digitalmodulator comprising: a differentiator to receive successive phasevalues at a sampling rate and provide differentiated phase values basedon the phase values; an adder to provide successive instantaneous phaseoffset values at the sampling rate based on both the differentiatedphase values and a frequency control word; a phase accumulator toprovide successive instantaneous phase values at the sampling rate basedon the instantaneous phase offset values; and an angle-to-amplitudeconverter to convert the instantaneous phase values to a multi-bitrepresentation of a phase modulated wave at the sampling rate.
 18. Thecircuit of claim 17, where the digital modulator further comprises: amultiplier to receive successive amplitude values and the multi-bitrepresentation of the phase modulated wave at the sampling rate; and themultiplier to output a multi-bit amplitude-and-phase-modulated signalthat changes in time according to the sampling rate.
 19. The circuit ofclaim 17, further comprising: a digital-to-analog converter (DAC) togenerate an analog modulated RF signal based on the multi-bitrepresentation of the phase modulated wave.
 20. The circuit of claim 19,further comprising: a digital up-conversion element operably coupledbetween the DAC and the angle-to-amplitude converter, where the digitalup-conversion element increases a frequency of the phase modulated wavefrom a first frequency to a second frequency.
 21. The circuit of claim19, further comprising: a resonant circuit coupled to an output of theDAC and adapted to filter undesired frequency components from the analogmodulated RF signal.
 22. The circuit of claim 18, further comprising: abaseband processor to provide both the successive phase values and afrequency control word to the digital modulator, where the frequencycontrol word is associated with a frequency channel over which the phasemodulated wave is to be transmitted.
 23. The circuit of claim 22, wherethe baseband processor provides the phase values in I-Q format, thecircuit further comprising: a cordic to convert the phase values in I-Qformat to phase values in polar format.
 24. A method of generating aradio-frequency (RF) signal in a transmitter, comprising: providing amulti-bit representation of a frequency control word, where thefrequency control word is associated with a carrier frequency; providinga multi-bit representation of data according to a sampling rate;providing a multi-bit digital modulated RF signal based on both themulti-bit representation of data and the multi-bit representation of thefrequency control word; converting the multi-bit digital modulated RFsignal into an analog RF signal; removing unwanted frequency componentsfrom the analog RF signal by using a resonant circuit.